Include raspberrypi.org GPG key in installer image

Before, the key was being downloaded insecurely with wget during
installation without verification. Now, the key that was downloaded and
verified by `update.sh` is included in the installer image so that it
doesn't need to be downloaded at install time.
Refine user interface of update.sh

Before, the output was very verbose, particularly with wget downloads.
Now, much less output is generated so that it is easier to monitor.
Additionally, added confirmation messages for successful verifications
and changed verification problems from warnings to errors.
Provide message if not all packages found in list

Before, `update.sh` would exit without any messages if not all required
packages were found in the package lists. Now, it tells the user what
went wrong.